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Topic: Best Light pollution map? (Read 1531 times)

Whether one considers any LP maps "not worthy substitutes", or not; if they help someone, no need to disregard them as a "mistakennumerical interpretation"! For my site, I never use the Bortle, rather using my SQM meter.

I think the members are well served with the different links being posted, and thanks to those posting the links.

Whether one considers any LP maps "not worthy substitutes", or not; if they help someone, no need to disregard them as a "mistakennumerical interpretation"! For my site, I never use the Bortle, rather using my SQM meter.

I think the members are well served with the different links being posted, and thanks to those posting the links.

Inaccuracy does not help anyone, it simply leads and misinforms them, Mr. Moderator. As I pointed out up-stream, the map cited is incorrect on both the bright and faint end of the colored regions, so just what can be gained from that as an observer?

Exactly to which maps are you referring? There are two styles of maps at the Website https://www.lightpollutionmap.info/ that give different and sometimes confusing information.

If you are referring to the VIIRS maps, be reminded that they do not represent sky brightness at all. They are maps showing upward radiance information ie the brightness of light emanating from each point on the ground. The color coding of that data bears no direct relationship to the more commonly used Bortle scale of estimating a combination of sky brightness at the zenith and the extent of light domes near the horizon.

If you are referring to the World Atlas 2015 map at the same Website, it is based on a best case calculated estimate of sky brightness at the zenith (without an actual separate inclusion of light dome visibility -- to the best of my understanding). In addition, the World Atlas 2015 map is based on data gathered in late 2014 with some data dating back to 2013 for selected areas. That is the latest data which has undergone the complex integration process used to generate the map.

Local "temporary" changes in sky brightness can occur due to forest fires, gas well flaring, construction projects, ground snow cover, and so forth. Some of these are accounted for by selective inclusion of data from the range of dates used as mapping data. Of course, any changes after 2014 will not be reflected in the maps until someone undertakes a new integration project using the latest data.

As you have pointed out, I think the Bortle scale was intended to account for overall sky conditions including both sky brightness at the zenith and extent and visibility of light domes at the horizon. To the best of my knowledge, no one has ever created a map that combines both pieces of information for a more accurate way to predict overall sky conditions at any given site.

The best we can do today is look to the zenithal brightness maps like the World Atlas 2015 map to get an overhead sky brightness estimate and then estimate light domes by referring to the VIIRS maps to show where most of the light will be coming from. I had attempted to do such a merging of data via a method I used when searching for a personal dark sky property a few years back. Using Google Earth, I was able to combine both forms of data using a methodology that included some calibration by taking panoramic photos at night from prospective sites. Unfortunately, Google moved the Google Earth program to the newer Web-App-based Google Earth Pro and my methodology quit working the same. I have not had time to try making things work reliably again in the new Google Earth Pro.John

A lot of folks use this one -https://www.lightpollutionmap.infoClick on the red Atlas in the menu in the upper right. Then when you click on various spots, it will give Bortle rating as well as the one I don't understand as much.

Unfortunately, this map system is quite inaccurate. I find that for New York and New England locations it typically provides values significantly too low on the Bortle scale for even modestly populated areas, while at the same time rating darker semi-rural locations as far too bright.

A lot of folks use this one -https://www.lightpollutionmap.infoClick on the red Atlas in the menu in the upper right. Then when you click on various spots, it will give Bortle rating as well as the one I don't understand as much.

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Unfortunately, this map system is quite inaccurate. I find that for New York and New England locations it typically provides values significantly too low on the Bortle scale for even modestly populated areas, while at the same time rating darker semi-rural locations as far too bright.

I like the GlobeatNight map, if we are looking at SQM readings over time. Here is a LINK to it. I'll probably add my historical data from Unihedron's website for my SQM-L model. My home is getting worse, and sites closer to home are getting worse. My personal favorite dark sites in Utah's West Desert are holding for now, but I expect over the next twenty years for those to begin to decrease as to maintain the SQM rate in the 21.8 range will require a longer drive out west. My favorite site I don't post is about 3 hours away in eastern Nevada. Darker than dark there still.

Actually, I'm not a Mr. and I was not posting as a Moderator, rather as a member that observes, and for my purposes, uses the SQM meter. For anyone that thinks the sites are not accurate, they don't have to use them; but those that like &amp; appreciate the sites, it probably suits their needs. We all have our opinion as to what is "accurate", when it comes to dark sky readings.

A lot of folks use this one -https://www.lightpollutionmap.infoClick on the red Atlas in the menu in the upper right. Then when you click on various spots, it will give Bortle rating as well as the one I don't understand as much.

Unfortunately, this map system is quite inaccurate. I find that for New York and New England locations it typically provides values significantly too low on the Bortle scale for even modestly populated areas, while at the same time rating darker semi-rural locations as far too bright.

A lot of folks use this one -https://www.lightpollutionmap.infoClick on the red Atlas in the menu in the upper right. Then when you click on various spots, it will give Bortle rating as well as the one I don't understand as much.

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Unfortunately, this map system is quite inaccurate. I find that for New York and New England locations it typically provides values significantly too low on the Bortle scale for even modestly populated areas, while at the same time rating darker semi-rural locations as far too bright.

BrooksObs[/quote]Which map do you recommend?[/quote]In all honesty, I can not recommend any specific map as none that I've have seen truly reflect the ground conditions an observer encounters at a given site, at least not east of the Mississippi. Typically, light pollution maps depict intensities about one or even two Bortle darkness classes too dark for specific locations in the eastern U.S., at the same time suggesting a more rapid drop-off in sky glow away from major cities than exists. The latter is particular true for any region that has a nearby significantly urbanized area, especially a large city. The glow of the New York Megalopolis is detectable well beyond 100 miles from Manhattan and to a small degree will still impact the sky's darkness nearly up to the zenith! Thus, while light pollution maps may depict such a location 100 miles out from NYC as Bortle class 2, it will most likely be class 3 or worse in reality.

The ONLY way to know the darkness of a location is to visit it on a good night and evaluate it yourself. Often one finds a compact nearby light source that is not indicated on the maps at all that causes the location to be a write-off. Larger service stations, rest spots, high school sports fields, mini-malls and such that are illuminate most, or all, night are surprisingly common across much of our country even in otherwise rural areas and they can often be seen at a considerable distance. Don't forget winter sport trail locations either. I've noticed these in winter from scores of miles away.

So, while light pollution maps may give you some approximation of how conditions are trending in an area, they are far from real predictors of any sort.

The reason I was asking is for that reason. They seemed to show different results. Some said I was in a yellow zone while some said I was in a light green light blue area which is a big difference.

If a green/blue zone is what you seek then concentrate on areas that show as green/blue on each or all of the maps. You'll have to verify from the ground, in person, whether the zone is yellow, green or blue. There is no rule that says that the LP maps must agree with each other.

A lot of folks use this one -https://www.lightpollutionmap.infoClick on the red Atlas in the menu in the upper right. Then when you click on various spots, it will give Bortle rating as well as the one I don't understand as much.

To which BrooksObs responded:

<p class="citation">QuoteUnfortunately, this map system is quite inaccurate. I find that for New York and New England locations it typically provides values significantly too low on the Bortle scale for even modestly populated areas, while at the same time rating darker semi-rural locations as far too bright....In all honesty, I can not recommend any specific map as none that I've have seen truly reflect the ground conditions an observer encounters at a given site, at least not east of the Mississippi. .[/quote]I agree that the map referred to above gives sky brightness readings that are consistently quite a bit darker than what I measure with my SQM.However, I find that my SQM readings are very much in line with David Lorenz's 2006 Light Pollution Atlas. This is the map referenced by the Clear Sky Chart.As for the correlation between SQM readings and Bortle classes, that's very problematic. I think that most of these are taken from the Wikipedia article on the Bortle scale, which I find quite clearly wrong on many fronts. Just to name one, putting all readings between 19.1 and 20.4 into a single Bortle Class (5) is lunatic. That's a huge range of different kinds of sky! SQM=19.1 is a poor suburban sky where the Milky Way is barely visible. At SQM=20.4, the Milky Way has considerable size and structure, and likely reaches from horizon to horizon unless it happens to hit the middle of one of the major light domes.

<p class="citation">QuoteThe ONLY way to know the darkness of a location is to visit it on a good night and evaluate it yourself. Often one finds a compact nearby light source that is not indicated on the maps at all that causes the location to be a write-off. Larger service stations, rest spots, high school sports fields, mini-malls and such that are illuminate most, or all, night are surprisingly common across much of our country even in otherwise rural areas and they can often be seen at a considerable distance. Don't forget winter sport trail locations either. I've noticed these in winter from scores of miles away.[/quote]All very true. Intermittent and seasonal light sources, such as playing fields and ski areas with night skiing, are particularly tricky, since an area that's fine when those lights are off can be unusable when the lights are on. Bright lights that are always on, such as truck stops, do in fact show up on the maps.

On the other hand, an area that's shown as unacceptably bright on any of the maps will almost certainly be unacceptably bright -- although there are highly localized sources of light, there's no such thing as a localized source of dark -- more's the pity.

My experience is that at least within any geographical area, such as the U.S. Northeast, the maps are pretty reliable for giving the relative brightness of different sites -- always assuming the absence of a local source that is too small or intermittent to be picked up by satellite. In other words, the red zone will always be brighter than the orange zone in any map using that color scheme, regardless of what absolute meanings you assign to red and orange. So when looking for a dark site near you, it makes sense to look first near the centers of areas that are shown as relatively dark on the maps.

Because of the proliferation of different maps using similar color schemes, it's getting almost meaningless to say "orange zone" without specifying which map you're using.

Forget all the LP maps for a moment and just look at the population densities of each county within about 200 miles of your prospective observing site. Pick the least densely populated counties and then within those counties find the spots that are furthest from their respective cities, towns and major highways. Then, weed out those with the worst potential sources of localized light pollution (not easy.)

Or just look for National Forests, mountain ranges, swamps or isolated seashores. Then there's the weather prospects....

Cheap, low-maintenance, low-operating-cost LEDs will complicate matters and make long-term development of dark sites, if you can find them anywhere, problematic in the coming years.

My thanks to the OP and to Jim. I have not seen that map before but really like the interactive it uses. My backyard at home is Bortle 4 like I already knew. That's the good news. The bad news is that there is a plant out near my weekend property / dark site that seems to be growing in brightness. It is still a Bortle 3 site though, so far...

Forget all the LP maps for a moment and just look at the population densities of each county within about 200 miles of your prospective observing site. Pick the least densely populated counties and then within those counties find the spots that are furthest from their respective cities, towns and major highways.

That works pretty well. The thing that the light-pollution maps include, which the above method does not, is a model for how light pollution spreads. For instance, the White Mountain National Forest in New Hampshire and Maine is a pretty big protected area with a few towns on inholdings inside it. Being resort towns, their nominal populations don't correlate very well with their degree of development. In addition, there are small cities to the north of the WMNF, modest-sized cities to the east and west, and cities ranging from big to huge to the south, but some distance away. The light-pollution maps give you a pretty good idea of how all those different sources balance out, and where within the WMNF might be best to look for the darkest sites.

My thanks to the OP and to Jim. I have not seen that map before but really like the interactive it uses. My backyard at home is Bortle 4 like I already knew. That's the good news. The bad news is that there is a plant out near my weekend property / dark site that seems to be growing in brightness. It is still a Bortle 3 site though, so far...

I find it interesting when there is a bright glow at some random spot in a reasonably dark zone. Feed the coordinates to Google Earth, and there is a big fat refinery/cement plant/etc sitting right there where it bothers only a few farmers/ranchers and those who seek darkness for legitimate reasons.